The development of very high energy permanent magnets has made possible the design of permanent magnet generators (PMG) which equal or surpass wound field generators' specific output (KVA per unit weight). This has made PMG's an attractive alternative to wound field generators for aerospace electrical generating systems where weight is always a prime consideration.
Other potential advantages of PMGs over wound field generators are the elimination of the rotor field power and the greater magnitude rotor magnetic fields which are attainable with PMGs when compared to magnetic fields produced by rotors of wound field generators. The higher magnitude rotor magnetic fields produced by a PMG permit higher efficiency. The elimination of the rotor field power and the exciters, rotating rectifiers and other apparatus for providing the rotor field power save weight, permit higher speeds of operation, lower manufacturing costs and higher reliability. A PMG has a significant weight advantage in systems in which the frequency of generation of electrical power is not constant such as 400 Hz. Known systems that do not generate constant frequency AC power in airframes are 270 volt DC and variable speed constant frequency (VSCF) electrical power generating systems which are driven directly by a variable velocity power takeoff from an airframe propulsion engine.
However, PMG's when compared to wound field generators, have a disadvantage regarding the control of the output voltage and protection against electrical faults. These disadvantages result from the fact that the permanent magnet field produced by the rotor is fixed and cannot be turned off as is the case with the wound field generator.
Output voltage control of PMG's can be achieved by using a switching electronic regulator. However, for a typical aircraft power levels this requires heavy and expensive switching devices. Furthermore, substantial switching losses reduce system efficiency. Lessened reliability results as a consequence of the complexity of the switching electronic regulator.
Voltage control for permanent magnet generators has been provided by multiple permanent magnet generators having either one or more rotors or one or more stators which are rotated with respect to one or more rotors or stators to produce a variable magnitude magnetic excitation field which produces the generated output potential. The output voltage is regulated by the variation of the phase angle between the multiple generator outputs produced in a common stator winding or by varying the relative angular position of the stators which are magnetically coupled to the permanent magnet fields produced by the multiple rotors. Adjustment of the angle between the plural rotors or stators permits the reduction of the output potential of the generator to zero. However, during normal operation, large magnetic fluxes from the permanent magnet rotors are magnetically coupled to the stator which prevents the de-excitation of the stator to protect against faults in the stator winding.
U.S. Pat. Nos. 3,713,015, 4,305,031, 4,371,801, 4,663,581, 4,728,841, 4,817,461, 4,879,484 and 4,882,513 and French Patent 2,007,310 are representative of the aforementioned prior art. These systems use actuators to rotate a rotor or stator to vary a relative phase angle between the generator rotors or stators to provide voltage control. These systems are complex, expensive and require an energy source to power the actuator.
In the prior art the only protection against internal winding faults for single or multiple rotor PMG's has been to mechanically disconnect the rotor from the source of rotational energy to bring the rotors to rest. A mechanical disconnect is interposed between the prime mover and the drive shaft of the rotor to permit the disconnection. In prior art PMG's with multiple rotors, variation of the relative phase angle between the rotors may reduce the output potential to zero but does not decouple the permanent magnetic fields from the stator when the output potential is reduced to zero.